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Chlorine : Hydrochloric Acid, Hydrogen Chloride |
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Chlorine and hydrogen form only one stable compound, hydrogen chloride, or hydrochloric acid, HCl. This occurs in some volcanic gases and in rivers near volcanoes. It is also found, to the extent of 0.2 to 0.4 per cent., in the gastric juice in normal conditions; in the dog's gastric juice as much as 3 per cent, is present. Hydrogen chloride is formed (Expt. 9) by the combustion of hydrogen in chlorine: H2 + Cl2 = 2HCl, and is produced technically in this way. It is usually prepared in the laboratory by heating common salt with concentrated sulphuric acid in a flask. The complete reaction (1) 2NaCl + H2SO4 = Na2SO4 + 2HCl requires a higher temperature than can be attained in glass apparatus and is achieved only on the technical scale. At moderate temperatures the reaction proceeds mainly according to the equation (2) NaCl + H2SO4 = NaHSO4 + HCl, so that the yield corresponds with about 70 per cent, of equation (1). The hydrogen of sulphuric acid can thus be displaced in two stages, with formation of acid salts and normal salts; hence sulphuric acid is called a dibasic acid. Hydrochloric acid, which contains only one atom of hydrogen, forms only one series of salts, the normal salts, and is called a monobasic acid. Expt. 16. - 25 gm. of dry common salt are placed in a 500 c.c. flask, and covered with 25 c.c. of concentrated sulphuric acid. A rapid evolution of gas occurs and when this slackens the flask may be gently heated. The gas is passed through a small wash-bottle containing concentrated sulphuric acid, and then collected in dry jars by downward displacement, since it is 1.27 times as heavy as air, and is very soluble in water. A convenient method of obtaining the gas is to use a Kipp's apparatus charged with concentrated sulphuric acid and lumps of salammoniac: a regular stream of gas is evolved.
It may be collected over mercury. When the jar is full of gas dense white fumes issue from the mouth, formed from the gas and atmospheric moisture, producing minute droplets of solution, which have a lower vapour pressure than the partial pressure of water vapour in the air. The dry gas is quite transparent. The gas should not be dried by phosphorus pentoxide, as this slowly absorbs it, 227 c.c. of dry gas being taken up by 1 gm. of pentoxide: 2P2O5 + 3HCl = POCl3 + 3HPO3. If the gas is passed into a flask of distilled water, kept cool by running water over the outside from a ring of perforated lead pipe placed over the neck, an aqueous solution of the acid - spirit of salt - is produced. Each bubble of gas dissolves as it leaves the delivery tube, and a considerable amount of heat is given out. The concentrated solution fumes strongly in the air. Hydrogen chloride is formed by the action of concentrated sulphuric acid on other metallic chlorides, such as potassium, ammonium, magnesium and calcium chlorides; but lead, silver, cuprous, mercuric and mercurous chlorides are acted upon only with difficulty, if at all. Hydrogen chloride is formed by the action of water on the chlorides of boron, aluminium, silicon and phosphorus (carbon tetrachloride, CCl4, is not acted upon by water): the very pure gas is best prepared by the action of water on silicon chloride, since that produced from sodium chloride and sulphuric acid contains traces of hydrogen sulphide: SiCl4 + 2H2O = SiO2 + 4HCl. |
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